The present invention relates to heaters for heating a stream of gas, such as relatively hot turbine exhaust gas or relatively cool flue gases.
It is well-known that exhaust gases in general and turbine exhaust gases in particular have a relatively high temperature. If such gases are discharged to the atmosphere, a large amount of energy is wasted. To effectively utilize the energy carried by such gases, say for the generation of steam, it has been proposed to heat the gases to raise their temperature. That steam can then in turn be employed to power steam turbines or for other advantageous uses. Some flue gases, on the other hand, need to be heated before they are discharged when the gases include chemicals which become corrosive below certain temperatures.
The term "exhaust gas" is used herein to designate a gas which typically has an elevated temperature, i.e. gas of a temperature higher than ambient temperature, and which further has an oxygen content less than that of air although the actual temperature of such gases and their oxygen content may vary widely. For example, turbine exhaust gases may have temperatures of as much as 500.degree. C. or more (and an oxygen content as high as 16% or more while scrubbed flue gases may have a temperature below 100.degree. C. and an oxygen content of as little as 2-3%.
In the past, exhaust gases have been heated in a variety of ways. The most inexpensive way to heat exhaust gases, at least as far as the construction of the heater is concerned, is to employ natural gas or light oil burners which are conveniently placed inside the exhaust gas duct. Examples of such heaters are disclosed in U.S. Pat. Nos. 3,632,286 and 3,830,620.
The increasing scarcity of gas and high quality, e.g. highly refined, light weight fuel oil has made it necessary to use heavy oils such as No. 6 fuel oil for the operation of gas turbines. This dictates that exhaust gases be heated with the same heavy oils. U.S. Pat. No. 3,934,553 illustrates such an exhaust gas heater. Briefly, it provides that the burners, including their fuel nozzles, be mounted exteriorly of the exhaust gas duct so that the fuel and the nozzle are never directly exposed to the hot exhaust gases. In this manner, a potential clogging of the fuel lines to the nozzle due to an excessive heating thereof by the exhaust gas is prevented. Thus, the flame is formed at the wall of the duct and is projected towards the center thereof into the flow of hot exhaust gas. To prevent the flame from being extinguished by the exhaust gas flow, a flame shield is positioned immediately upstream of the burner so as to form a trough within which the flame can burn in a manner analogous to protecting a candle from being blown out by shielding it with one's arched hand against air drafts.
For maximum efficiency, it is desirable that as little outside air as possible be introduced into the duct to sustain the flame since such outside air proportionally cools the gas flow and since the purpose of the heater is to raise the exhaust gas flow to the desired level at which the heat energy in the gas can be used to generate steam, for example. Accordingly, the burners appear to operate with relatively low primary air, i.e. outside air mixed with the fuel in the burner and the U.S. Pat. No. 3,934,553 patent discloses to perforate the shield by including holes and passages therein which permit the flow of part of the exhaust gas "through" the shield to the flame so that combustion oxygen for the flame can be extracted from the exhaust gas.
A difficulty with this approach is that the burner becomes quite unresponsive to regulation, that is if the perforations in the shield are formed so as to provide the flame with sufficient oxygen for maximum operation, the perforations typically flow an excessive amount of exhaust gas to the flame when it operates in a turn-down mode. In fact, at that point, too much exhaust gas may penetrate the flame shield and the flame may become extinct. Thus, such heaters are not well adapted for use over a wide operating range.
Moreover, heaters of the type disclosed in the U.S. Pat. No. 3,934,553 have a tendency to unevenly heat the exhaust gas so that the gas downstream of the heater may exhibit hot spots which, in turn, may lead to a local overheating of the heat exchange surfaces over which the gas subsequently flows. Such uneven heating is the result of the provision of spaced apart shields which are formed so as to define a protective trough for a particular portion of the flame, typically its base proximate the burner where the flame is the widest. As the flame narrows towards its end, its tranverse dimension becomes less and less, yet the protective shield forms a barrier with the same cross-section as in the vicinity of the flame base. As a result, exhaust gas streaming through the center of the duct is heated relatively less than exhaust gas streaming past the sides of the duct on which the burners are mounted so that the center portion of the gas stream may become less heated than the peripheries thereof. This can adversely affect the overall operation of the duct heater and the associated heat exchange surfaces.
Thus, the most recent prior art exhaust gas heater seeks to devise a heater which can be operated with lower grade, heavier fuel oils instead of with the much more expensive and increasingly scarce light weight fuel oils and/or fuel gas. To avoid the clogging of fuel lines from the coking of the overheated heavy fuel oils, the burners were essentially mounted outside the exhaust gas duct and shields were provided to protest the flames.
Although flame shields of this type in general are nothing new and were previously employed to protect the flames of gas fired duct heaters, as is disclosed in U.S. Pat. Nos. 3,494,712 and 3,649,211 to Vosper and assigned to the assignee of the present application, the flame shields employed in connection with exhaust gas heaters of the type described in the above-referenced U.S. Pat. No. 3,934,553 simply constituted shields which were formed with only one function in mind, namely to serve as an anchor for the flame in the exhaust gas stream so as to prevent it from being blown in a downstream direction. However, for an efficient operation of the burner and a minimization of atmospheric pollution more is required of such shields since the shields, when placed in an exhaust gas stream, act as baffles or guides for the exhaust gas which channel the gas along numerous paths essentially about and past the flames of the heater. Thus, the shields can induce eddies on their downstream side which, if not controlled, can lead to an accumulation of carbon, soot and the like which can ultimately be discharged to the atmosphere and cause pollution; the shields determine how close the various exhaust gas streams come to the flame and, thereby, how evenly or unevenly the gas will be heated which, if not controlled, may lead to hot spots in certain portions of the gas flowing downstream of the heater and thus may damage heat exchange surfaces located there; and, most importantly, the shield and the above-discussed perforations determine to what extent and how combustion oxygen for the flames of the heater from sources other than outside air is supplied to them--in this regard, closest control is necessary if a complete and efficient combustion of all fuel is to be assured during all operating conditions of the burner.
The exhaust gas heater of the U.S. Pat. No. 3,934,553 does not take into account these aspects. As a result, the heater disclosed in the U.S. Pat. No. 3,934,553 patent is only fully satisfactory insofar as it is capable of heating the exhaust gases with heavy fuel oils without requiring the frequent cleaning of the burner and in particular its fuel supply lines. Its operating characteristics, operating range and efficiency, however, are less than fully satisfactory. Thus, there is presently a need for an exhaust gas heater capable of using heavy fuel oils which eliminates or at least significantly reduces the drawbacks encountered with prior art heaters of this type.
In addition, the exhaust gas heater disclosed in U.S. Pat. No. 3,934,553 relies on the oxygen in the exhaust gas stream for a substantial portion of the combustion oxygen required by the flame of the burner. Although sufficient combustion oxygen is normally available from turbine exhaust gases, that is not the case with other types of exhaust gases such as flue gases which may have a little as 2 to 3% of oxygen. In such instances, the combustion oxygen must be provided by combustion air, both primary air introduced by the burner and secondary combustion air introduced over the length of the flame. Since the environment within which the flame burns is effectively devoid of oxygen, it is difficult to achieve complete combustion. Yet, incomplete combustion leads to the discharge of pollutants which is unacceptable under today's strict pollution control laws and regulations. Duct burners capable of operating under such conditions are presently unavailable.